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secret modulation input revealed open up the pod to a whole new world of modulation! (without necessarily having to open the pod) first, the obvious: i`m happy with the pod as is. the pod has plenty modulation built in already: - almost all effects have their independent modulator/LFO with speed and depth settings. - mic input can modulate the vocoder - internal and external pedal can be assigned to almost everything. these are all fine and can go a long way. however, there is mostly no LFO waveform setting, and they cannot be linked or synchronized. far as i know, midi is poorly implemented in the pod hd series and basically has switching functionality only. if you are like me and like to get a little more experimental (and have some experience with soldering), read on: i will be taking it in small steps starting from non-invasive and going on to daring/hazardous. its not too hard, really. cost is marginal, compared to the reward: a few $ or € will have you connecting extra modulation with very little effort. . possible modulation source: - modulation output from anything providing a CV (control voltage) or pwm like synths, sequencers etc. - midi to cv interface (lets you freely draw the modulation curve in your daw, how cool would that be? just imagine!), over midi or usb. wish the edit program could be automated by a vst host, changing modulation target. - dedicated modulation source, like Lfo section of older modular analog synths, e.g. formant system (which is what i am using, extended version with 4 lfo, some 20 dip switches and mixing stage, 16 outputs), circuitry is not all that complicated. 1 quad opamp per lfo does it, can easily be expanded/modified. documentation is available. sections are independent. link: http://www.sequencer.de/forumsynth/forumsynth.html#formant any other lfo will do, results may vary. ________________________________________________________________________________________ now, the disclaimer: please excuse my english, not my 1st language. always pull the plug when messing with electrical appliances. anything you do to or with your pod is at your own risk, please dont blame me. things can go wrong. you have been warned. if in doubt, dont get started. get a savvy friend to help you or a second opinion. for all of the below, i have tried to make sure things are failsafe: even in a worst case scenario, they may not work but should not do any damage or adversely affect the pod permanently either. (if applied properly. only you can do the damage. people in Fukushima may have a different attitude towards failsafe, but were not dealing with rocket science here). where there is a risk i will warn. one real danger is probably from statics at the pod input and semiconductors in general, so always keep everything on the same outlet, which will also help reduce general noise issues. plug away before switching on (just a recommendation. i know noone really does it). in case of EXP2, you do need to plug in before powering up, else it wont be recognized. if the modulators cannot be disabled it may help to bring the pedal to heel position which will short any parallel input. try to stay grounded, especially when opening the pod. same goes for your soldering equipment. the exp in a/d does not detect the pedal`s ohm value directly. instead, a current is fed into the pot resulting in a voltage which can easily be converted. the pedal 2 input expects voltages from roughly 1.75V toe to 0.0V heel position. see if your modulation source sends output of > 5V, or negative output. resistor and pot values may need to be adjusted, or some op amps and/or diodes may need to be added to make up or level output range. these will follow in the more advanced parts of this article. always use strong diodes like 1n4007, they will be able to handle any malfunction or overload. i will be using 1 diode in reverse which will direct any voltage below -0.7V to ground, and 2 or 3 diodes forward in series to cut any voltage above 1.x to 2.1V. alternatively, a single 1.5 - 2.x V zener diode could do both, but isnt as sturdy. the diodes in turn are being protected by resistor(s) where needed. if your modulation source is not adjustable, it may hit the ceiling and floor given by these diodes, making changes to resistors necessary so the mod signal stays within bounds. all below non-invasive steps (without removing the pull up resistor inside the pod) are a compromise due to the low ohm pedal pot. they can either be plugged into the pod hd pedal 2 input as standalone or with the pedal 2 attached in parallel or series. a simple y-adapter will let you use modulation and pedal in parallel, else the modulation input and circuitry could be fitted inside the pedal 2. The EXPx a/d has its limitations. LFO frequencies > 10Hz will not work and may give unexpected results (artefacts) due to the low sample rate of the conversion, 500x users might get better results due to dsp headroom please note: pedal calibration of the internal pedal may require to temporarily remove below options except where explicitly stated) if the modulation source cannot be muted. there is no pedal calibration for EXP2. removal is easy for all non-invasive options please note the difference between pod and pot: where pod is the pod hd, pot is a potentiometer. there are 2 kinds of pot mentioned here: the actual pedal pot and a pot to limit/set the modulation voltage or set the current for a led, op-amp or fet. since i dont have access to a pod hd X model, i can only assume the pedal input circuitry hasnt been changed much. not sure if it has a EXP2 input at all in general, any device and brand with a pedal or other mod input can be used with these circuits, with some changes to resistor values. __________________________________________________________________________ roadmap: non-invasive, input modulation via EXP2, using: LDR resistor/capacitor op-amp invasive: double your new modulation input capabilities: all above measures can be translated to apply for internal pedal 1 just the same. use EXP1 simultaneously and independently by giving it an extra input jack (or rewire one existing jack you wont use otherwise) remove pull-up from EXP1 and EXP2, making it easier to level your modulation source to the pod EXPx a/d range. to do: diagrams and schematics ______________________________________________________________________________________ ________________________ 1) non-invasive options: for starters, the external pedal 2 input EXP2 is well suited, except for its low ohm value. the pod feeds a current into the pedal`s 10k pot resistor. the resulting voltage is a/d`d and that value applied to any assigned effect (max. +1.75V). setting the pedal to heel position is more or less shorting the input which is worst case, so this is an expected and legit state, no danger here. none of the procedures will actually physically short the input ever, other than the pedal in heel position. the following can be used standalone even if you dont own the external pedal, or in parallel with an existing pedal, allowing for even better control. - y-adapter: one 1.4" plug, 2 sockets can take the pedal in parallel, easily removable. - or use a cable from inside the pedal/pot with extra plug/socket as modulation input. a dummy plug/socket can be used to protect it when not in use. in this case the pot in the pedal could be used in parallel or in series. ______________________________________________________________________________ ________________ 1: using an LDR (light dependent resistor) without modulation input: really passive, for the faint of heart, no danger whatsoever. a real starter`s project with nothing required except some wire or cable with a 1/4" plug, and 1 (or more) LDR, and the soldering. optionally your 1/4" y-adapter if using the exp2 pedal in parallel, or a 10k resistor to trick the pod into thinking there`s a pedal there (in virtual toe position, if/while the ldrs are kept in the dark). LDRs are not polarized. it doesnt matter which end goes to ground, nothing to consider. in real life, ldrs need a protective resistor. i use the pull-up resistor inside the pod which feeds the exp.2 pot. LDRs are in the M-ohm range when in the dark, and can get as low as k-ohms or even less when exposed to light. the amount of light in your shady shag will not cut it though. if you have a lighting crew youre in luck, else get it in front of a strong light source and move your hand over it for shade. could be mounted on your guitar for easy reach, the mechanics are off my topic though. then the resistance can be changed by moving it in and out of the light, by moving the axe, or by dancing in front of it. since infrared is similar to light, only @ a different wavelength, the ldr will also respond to temperature changes from outside, as well as to those from the current running through it. there are some pretty good looking ldrs out there, more like gems than electronics devices. if you want more info on ldrs, there are plenty good articles on the web, with diagrams for different types. you want as lo-ohm as possible, and probably many. one thing i did a while back is use the reflector of a flashlight to collect light, mount the ldr in the position of the bulb, and stick it all to a headband. nod away while youre noodeling... advantages: absolutely foolproof, no modulation source required. no external voltage involved. 1 LDR, one wire from pedal to LDR (if it can be grounded on the other end, like the guitar, else: 2 wire), 1 10k resistor if used without the pedal in parallel. worst case: the current from the pod avalanches the ldr in which case it may be beyond recovery, shorting the exp2 input. unplug, no damage done. of course we dont expect the current to be strong enough to blow the ldr, we are on the safe side and well below the danger zone by a factor of at least >20. at low voltages, the ldr can take up to 20-30mA. in theory, ldrs will produce a voltage when exposed to light. this effect can be disregarded in this application. disadvantages: ldrs are generally hi-ohm and need a lot of light to produce like 10k or less. you may need several (or more) in parallel to get good results, below 1k ohm. results will vary and may be hard to reproduce, depending on the ambient light intensity. a lens could help to collect and focus light on it. if used standalone, you need to put a 10 k resistor in parallel to the ldr or the EXP2 input may not get recognized and switched off. you will not get full heel position zero ohms with this simple circuitry. not for the purists (the whole project isnt for you, i guess): if you are willing to add 2 capacitors, one to your guitar out and one to the other end of the guitar out, or an extra cable, the ldr could be mounted on the guitar and fed through just 1 mono or stereo cable while also taking the audio from the same cable. __________________________________________________________ 2: using an LDR with external modulation input: (the real thing) if manually modulating is not for you, while youre busy playing, here`s an equally safe way to start. using an LDR (light dependent resistor) with LED as optical interface: ldrs have been used in effects early on and still are, e.g. opto compressors, opto tremolo. they are inexpensive and reasonably reliable and come in all shapes and sizes. anything analogue could be controlled or replaced by ldr. advantages: - there is no electrical connection between the pod and the modulation source at all, only light, which cannot do any damage. very safe, as long as the separation of modulation/led input and ldr output is not compromized. even if it were, there are no dangerous voltages involved. both signal and ground are kept separate. - only few parts required: ldr, led, a couple resistors, pot, diodes, wire, jack/plug, box, optional activity led. could be built in less than an hour. box being a matchbox type housing which keeps surrounding lightsources from influencing the ldr unless intended. disadvantages: - needs a battery or other psu unless the source (the lfo, interface) is able to drive a led (20mA @ 5V`= 250 ohms. the formant lfo can. if using a usb midi cv interface, supply voltage could be taken from there for a led driver (just a little invasive, but not at the pod end.) - ldr curve is not linear - ldrs tend to be high-resistance. zero values are impossible to reach (without damaging the component: if it has zero resistance, it needs replacement). you could use 2, a dozen, or more in parallel to improve response and range. use mirrors inside the box to save leds - ldrs are temperature sensitive, a bridge type cirquit may be required for compensation, - results may vary depending on the environment and may only be poorly reproduceable circuitry is pretty much straightforward and basic: input jack from modulation source with in ground --- pot (to set operating min/max values and operating current for the led) --- resistor --- led --- source ground output jack/plug from LDR --- pod pedal 2 in pod ground to other end of ldr (polarity doesnt matter to the ldr) values of resistors and pots will depend on the source`s output resistance and voltage, and the type of ldr, since the pedal 2 cannot be calibrated in the pod. except for what is described in the following paragraph "not for the purist", the led/ldr combination has to be encapsulated to avoid any ambient light other than that from the led to affect the ldr, unless intended. you need a 10k resistor in parallel with the ldr, or the EXP2 pedal in parallel so the pod can recognize it. r4 r5 -- -- opt. 5V in )---l__l--l__l---- pot l LDR pedal r1 __ -- l r2 r3 ext. mod in )---l__l--l__l---- ---------------------------( to pod pedal 2 in I I r6 l l l l l l_ l _ _ / V ---> l l l l/ led ___ ---> l_l l_l l l l ---( to EXP2 gnd l l l l )- l l l l l l === === === ext. gnd -=- -=- pod gnd r3 is the pot in the pedal, or a 10k resistor if used without a pedal r2 is the LDR r1: a value of 200-250 ohms for each led is good for 2V to 5V input range, depending on the led type, output resistance of the mod source. pot r6 is around 100 ohms the type of led needs to be taken into consideration: leds differ in voltage (1,4 to 5V), current (10-40mA) and light intensity where green and white are probably brightest if input voltage goes below its working voltage, the led needs to be supplied (additional 5V input and resistor/pot r4, r5 required in parallel to r1). needs to be adjusted so the led will just barely be off when the input is zero or @ it`s lowest. negative input is possible/allowed. the led current needs to be in middle then, allowing to go from maximum bright to completely dark. if many ldrs are required, a light bulb may be better suited, it radiates in all directions while a led emits ln 1 direction only. bulbs tend to afterglow, further limiting max. modulation speed. diodes on the pod pedal2 input are not really necessary here, since there is no electrical connection from input to output, neither signal nor ground connection. only light passes from led to ldr as symbolized by the 2 arrows ---> the input could even be driven by an audio signal. ldrs have a typical reaction time in the milli second range. audio would be rectified by the led, the upper half of the signal ends up as an envelope at the ldr. could use a capacitor across the ldr to buffer, 100nF will do. _________________________________________________ more to come soon... if any of you are interested